The present invention relates to a process for producing difluoromethane (hereinafter, also referred to as xe2x80x9cHFC-32xe2x80x9d) having a high purity by removing hydrogen fluoride (hereinafter, also referred to as HF) from a mixture of HFC-32 comprising HF, for example a reaction product comprising HFC-32 and HF which product is prepared by a production process for HFC-32 by means of fluorination of dichloromethane (hereinafter, also referred to as xe2x80x9cHCC-30xe2x80x9d).
Recently, the ozone layer depletion of the stratosphere by means of chlorofluorocarbons has been a serious problem, and the uses thereof are prohibited internationally. Further, productions and uses of hydrochlorofluorocarbons are also restricted. HFC-32 as a compound free from chlorine has an ozone layer destruction factor of zero and thus its global warming factor is small, and has a good freezing capacity, so that HFC-32 is said to be favorable as an alternative cooling medium in place of the chlorofluorocarbons which are restricted.
In order to remove an acidic material, such as HF from the halogenated hydrocarbons, a process would be generally considered, in which the content of such a material is decreased as low as possible through a procedure, for example distillation procedure before washing with water. However, in many case, it is not easy to remove HF from the halogenated hydrocarbons to an extent that the remaining halogenated hydrocarbons may be washed with water since the halogenated hydrocarbons often form a minimum azeotropic mixture with HF.
As a process to remove HF from the azeotropic mixture comprising halogenated hydrocarbons and HF, a process wherein the mixture including HF is chilled to let each component of the mixture separate from each other in liquid phase or layer (so-called liquid phase-liquid phase separation), the upper phase (or layer) mainly comprising HF being recycled to the reaction step, and the lower phase (or layer) mainly comprising halogenated hydrocarbons being distilled to obtain a halogenated hydrocarbons including substantially no HF is disclosed in, for example, U.S. Pat. Nos. 2,640,086; 4,209,470; 5,094,773; EP-A No. 04 67 531 or Japanese Patent Kokai Publication No. 5-178768.
U.S. Pat. No. 3,873,629 discloses a process for continuous separation of HF from chlorodifluoromethane, wherein the gaseous mixture of these two components are counter-currently contacted with sulfuric acid to remove HF.
In addition, U.S. Pat. No. 3,976,447 proposes a process to obtain a halogenated hydrocarbons containing no HF through absorption by particles of calcium chloride, barium chloride or strontium chloride.
Japanese Patent Kokai Publication No. 7-258125 discloses a process for removing HF through two-stage distillation with the azeotropic composition of HFC-32 and HF varying by changing pressure thereof.
The system consisting of HFC-32 and HF does not substantially have a phase-separation point under a commercially operatable temperature condition, which is generally above around xe2x88x9230xc2x0 C., due to mutual dissolution of each components, so that a process using phase separation is not applicable to such a system.
In addition, this system forms a minimum azeotropic mixture. However, since the HF concentration in such an azeotropic composition is too low, a considerable amount of HFC-32 is required to be distilled as an azeotropic mixture with HF in order to remove HF by means of distillation. As to the azeotrope of the system consisting of HFC-32 and HF, International Patent Publication No. WO93/21140 may be referred (the disclosures of this patent Publication is herein incorporated by reference). The distilled HFC-32 is necessary to be recovered from the azeotropic mixture with HF since the amount thereof is considerably large. Therefore, a process to wash the azeotropic mixture with water to remove HF may be considered.
However, the resultant HFC-32 may contain water in an amount up to around the saturation point due to such a water-washing treatment. In order to solve such a problem, use of dehydrating agent comes to be required. Then, use of dehydrating agent for halogenated hydrocarbons yields another problem that the dehydration performance is not so sufficient, and in addition, HFC-32 may be decomposed depending on the kind of the dehydration agent. Further, it is necessary to be considered that HFC-32 is mainly used as a refrigerant, and HFC-32 having a high purity with little water content is desired. When these factors are considered, removing HF from HFC-32 by means of the combination of azeotropic distillation with water washing is not always a preferable process.
The process to vary the azeotropic composition by change of pressure is also not an effective process for the system consisting of HFC-32 and HF since the HF concentration in the azeotropic composition is too low to be required to recycle a large amount of HFC-32 and energy loss due to heating and cooling therethrough is too large.
Thus, the conventional processes for removing HF as described above is neither industrially applicable nor economical.
The above U.S. Patent process in which the mixture of the two components are contacted with sulfuric acid describes that the HF concentration in the chlorodifluoromethane may be decreased from about 3.0% by weight to about 0.2% by weight. However, it is not clear from the disclosure of the aforementioned U.S. Patent whether or not the process may be industrially and effectively applicable to the case addressed by the present invention wherein HFC-32 has less initial HF concentration, for example 0.5% by weight or less since the HF concentrations and the kinds of the fluorohydrocarbons are different from each other.
Therefore, it is an object of the present invention to provide a process for producing HFC-32 wherein the HF removal from HFC-32 which has been said to be difficult as described above is carried out effectively, and in other words to provide a process for refining HFC-32.
As a result of the inventors"" intensive research on the process for refining HFC-32 to effectively remove HF from HFC-32, which has been said to be difficult, the present invention is accomplished.
The present invention is a process to obtain a mixture comprising HFC-32 and reduced content of HF, preferably substantially no HF by means of contacting a mixture comprising HF and HFC-32, for example a mixture having the azeotropic composition thereof (e.g. having a composition wherein the weight by weight ratio of HF/HFC-32 is 1/99 under a temperature of 7.2xc2x0 C. and at a pressure of 10.2 kg/cm2-abs.) with sulfuric acid,
characterized that the HF concentration in the mixture before contact is less than about 1% by weight, preferably less than about 0.5% by weight, the operating pressure at the contact is from 10 to 40 kg/cm2-abs. and the operating temperature is within a range from 10 to 100xc2x0 C.